Liquid crystal flat panel display with enhanced backlight brightness and specially selected light sources

ABSTRACT

Brightness in the LCD is enhanced by polarization recycling using a pre-polarizing film to pre-polarize light, and a special reflector for recycling light reflected by the pre-polarizing film. In one embodiment, the pre-polarizing film comprises a layer of DBEF brightness enhancement film, and the rear reflector is made of a PTFF material. In another embodiment, the rear reflector is covered with a film comprising barium sulfate. The multiple light sources are selected such that, at any color temperature within a predetermined range, the brightness of the LCD is not reduced below a given threshold minimum (e.g., 70 percent of the maximum brightness). Another constraint for selecting the light sources is that within the predetermined color temperature range, the color temperature is held close to the black body curve of the CIE chromaticity diagram. The light sources are also selected so that their maximum brightness point is set to be near the middle of the predetermined color temperature range.

This is a continuation of application(s) Ser. No. 09/087,280 filed onMay 29, 1998, now U.S. Pat. No. 6,243,068.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to the field of display devices. Morespecifically, the present invention relates to the field of flat paneldisplay devices utilizing liquid crystal display (LCD) technology.

(2) Prior Art

Flat panel displays or liquid crystal displays (LCDs) are populardisplay devices for conveying information generated by a computersystem. The decreased weight and size of a flat panel display greatlyincreases its versatility over a cathode ray tube (CRT) display. Highquality flat panel displays are typically back-lit. That is, a source ofillumination is placed behind the LCD layers to facilitate visualizationof the resultant image. Flat panel LCD units are used today in manyapplications including the computer industry where flat panel LCD unitsare an excellent display choice for lap-top computers and other portableelectronic devices. However, because the technology of flat panel LCDunits is improving, they are being used more and more in othermainstream applications, such as desktop computers, high-end graphicscomputers, and as television and other multi-media monitors.

In the field of flat panel LCD unit devices, much like conventionalcathode ray tube (CRT) displays, a white pixel is composed of a red, agreen and a blue color point or “spot.” When each color point of thepixel is illuminated simultaneously, white can be perceived by theviewer at the pixel's screen position. To produce different colors atthe pixel, the intensities (e.g., brightness) to which the red, greenand blue points are driven are altered in well known fashions. Theseparate red, green and blue data that corresponds to the colorintensities of a particular pixel is called the pixel's color data.Color data is often called gray scale data. The degree to whichdifferent colors can be achieved by a pixel is referred to as gray scaleresolution. Gray scale resolution is directly related to the amount ofdifferent intensities to which each red, green and blue point can bedriven.

The method of altering the relative color intensities of the colorpoints across a display screen is called white balance adjustment (alsoreferred to as color balance adjustment, color temperature adjustment,white adjustment, or color balancing). In other words, the appearance of“white” is a combination of red, green and blue intensities in variouscontributions of each color. “Color temperature” attempts to correlatethe temperature of an object with the apparent color of that object. Itis the temperature of the light source that illuminates the object.Ideally, that source is a perfect black body emitter, e.g., a thermallyradiating object that absorbs all incident radiation and re-radiatesthat energy with complete efficiency. A theoretical model of such ablack body was derived by Max Planck and is the standard to which anythe source is compared.

But real life radiators are not so efficient but still tend to followPlanks equation in a relative sense and are known as “gray body”emitters. A tungsten filament is a very good approximation to a graybody and the user of a tungsten filament as a substitute for a blackbody reference is wide spread. Therefore, the term “color temperature”refers to the emission spectra of a tungsten filament t a giventemperature as expressed in degrees Kelvin. In a display, the “colortemperature” of white correlates to the relative percentagecontributions of its red, green and blue intensity components.Relatively high degree K color temperatures represent “white” having alarger blue contribution (e.g., a “cooler” look). Relatively smalldegrees K color temperatures represent “white” having a larger redcontribution (e.g., a “warmer” look). Generally, the color temperatureof a display screen is adjusted from blue to red while avoiding anyyellow-ish or green-ish variations within the CIE chromaticity diagram.

The white balance adjustment for a display is important because manyusers want the ability to alter the display's color temperature for avariety of different reasons. For instance, the color temperature mightbe varied based on a viewer's personal taste. In other situations, colortemperature adjustment may be needed to perform color matching (e.g.,from screen-to-screen or from screen-to-paper or screen-to-film). Insome situations, color temperature adjustment can correct for theeffects of aging in some displays. Therefore, it is important for a flatpanel LCD unit to provide the user with a color balancing adjustmentoption.

One method for correcting or altering the color balance within an LCDunit screen is to alter, on-the-fly, the color data used to render animage on the screen. For instance, instead of sending a particular colorpoint a color value of X, the color value of X is first passed through afunction that has a gain and an offset. The output of the function, Y,is then sent to the color point. The function is specifically selectedfor a particular color temperature result. The values of the abovefunction can be altered as the color temperature needs to be increasedor decreased in value. Although offering dynamic color balanceadjustment, this prior art mechanism for altering the color balance isdisadvantageous because it requires relatively complex circuitry foraltering a very large volume of color data. The circuitry adds to theoverall cost of production and can increase image generation latency.Secondly, this prior art mechanism may degrade the quality of the imageby reducing, e.g., narrowing, the gray-scale range and therefore thegray-scale resolution of the flat panel display. Therefore, it isdesirable to provide a color balance adjustment mechanism for a flatpanel display screen that does not alter the image data nor compromisethe gray-scale resolution of the image.

Another method of correcting for color balance within a flat paneldisplay screen is used in active matrix flat panel display screens(AMLCD). This method pertains to altering the physical color filtersused to generate the red, green and blue color points. By altering thecolor the filters, the color temperature of the AMLCD screen can beadjusted. However, this adjustment is not dynamic because the colorfilters need to be physically (e.g., manually) replaced each timeadjustment is required. Therefore, it would be advantageous to provide acolor balancing mechanism for a flat panel display screen that canrespond, dynamically, to required changes in the color temperature ofthe display.

Within CRT devices, color balancing is performed by independentlyaltering the voltages of the primary electron guns (e.g., red, green andblue guns) depending on the color temperature desired. However, like theprior art mechanism that alters the color data on-the-fly, this priorart color balancing technique reduces the gray-scale's dynamic range andtherefore the gray-scale resolution of the display. Also, this techniquefor color balancing is not relevant for flat panel LCD units becausethey do not have primary electron guns.

Accordingly, the present invention offers a mechanism and method forproviding color balancing within a display that does not require a largeamount of complex circuitry and does not reduce the gray-scaleresolution of the display. Further, the present invention offers amechanism and method that dynamically alters the color balance of adisplay and is particularly well suited for application with flat panelLCD units. These and other advantages of the present invention notspecifically described above will become clear within discussions of thepresent invention herein.

SUMMARY OF THE INVENTION

Multiple light source systems are described herein for color balancingwithin a liquid crystal flat panel display unit. The present inventionincludes a method and system for altering the brightness of two or morelight sources, having differing color temperatures, thereby providingcolor balancing of a liquid crystal display (LCD) unit within a givencolor temperature range. The embodiments operate for both edge andbacklighting systems. In one embodiment, two planar light pipes arepositioned, a first over a second, with an air gap between. The lightpipes distribute light uniformly and independently of each other. Thefirst light pipe is optically coupled along one edge to receive lightfrom a first light source having an overall color temperature above thepredetermined range (e.g., the “blue” light) and the second light pipeis optically coupled along one edge to receive light from a second lightsource having an overall color temperature below the predetermined range(e.g., the “red” light).

In the above embodiment, the color temperatures of the first and secondlight sources are selected such that the overall color temperature ofthe LCD can vary within the predetermined range by altering the drivingvoltages of the first and second light sources. In effect, the LCD colortemperature is altered by selectively dimming the brightness of one orthe other of the light sources so that the overall contribution matchesthe desired LCD color temperature. In the selection of the lightsources, a constraint is maintained that at any color temperature thebrightness of the LCD is not reduced below a given threshold minimum(e.g., 70 percent of the maximum brightness). In the selection of thelight sources, a second constraint is maintained that within thepredetermined color temperature range, the color temperature is heldclose to the black body curve of the CIE chromaticity diagram. In athird constraint, the light sources are selected so that their maximumbrightness point is set to be near the middle of the predetermined colortemperature range.

In furtherance of one embodiment of the present invention, the lightsources selection process may be implemented in computer readableinstructions executable by a computer system and stored in computerreadable memory such that a large number of number of light sourcescandidates may be simulated to obtain their luminance, chromaticity, andcolor temperature data. Candidates that satisfy the above constraintsare selected. In one embodiment, the selection process includes the stepof analyzing high-resolution spectral files of R, G, and B phosphors,varying a percentage composition of the R, G, B phosphors to generatemultiple sets of light source candidates, matching up the light sourcecandidates to generate a pool of candidate pairs, calculating a colortemperature-luminance relationship for each candidate pair, andrejecting the candidate pair unless the color temperature-luminancerelationship satisfies the above predefined selection constraints.

In another embodiment of the present invention, the selection processspecifically includes the steps of calculating a chromaticityrelationship for each candidate pair and rejecting the candidate pair ifthe chromaticity relationship deviates significantly from the black bodycurve. In yet another embodiment of the present invention, the selectionprocess further includes the steps of examining the colortemperature-luminance relationship to determine whether a peakbrightness point occurs at the middle of the given color temperaturerange.

One embodiment of the present invention includes a color balancingsystem within a flat panel display for providing color balancing withina color temperature range, the color balancing system having: a firstplanar light pipe disposed to provide backlight to a liquid crystaldisplay (LCD) layer; a first light source optically coupled to providelight to the first planar light pipe, the first light source having acolor temperature that is below the minimum color temperature of thecolor temperature range; a second planar light pipe disposed parallel tothe first planar light pipe such that an air gap exists between thefirst and the second planar light pipes, the second planar light pipealso for providing backlight to the LCD layer; a second light sourceoptically coupled to provide light to the second planar light pipe, thesecond light source having a color temperature that is above the maximumcolor temperature of the color temperature range; a pre-polarizing filmdisposed between the light pipes and a rear polarizer of an LCD; and arear reflector positioned on the other side of the light pipes. Thesystem also has a circuit coupled to the first and the second lightsources for setting a color temperature of the flat panel display byselectively and independently varying the brightness of the first lightsource and the brightness of the second light source. The circuitdecreases the brightness of the first light source to increase the colortemperature of the flat panel display and decreases the brightness ofthe second light source to decrease the color temperature of the flatpanel display.

Significantly, in the present embodiment, brightness in the LCD isenhanced by polarization recycling. According to the present embodiment,the pre-polarizing film first pre-polarizes light emitted from the lightpipes to a predetermined orientation that matches the polarizationorientation of the rear polarizer of an LCD. Light that is not polarizedin the predetermined orientation is reflected by the pre-polarizing filmto the reflector where it is rephased to the predetermined orientation.Consequently, brightness of the LCD screen is significantly improved bythe recycling. In one embodiment, the pre-polarizing film comprises alayer of DBEF brightness enhancement film, and the rear reflector ismade of a PTFF material. In another embodiment, the rear reflector iscovered with a film comprising barium sulfate. Brightness may also besignificantly enhanced by the addition of a crossed BEF layer betweenthe rear polarizer of the LCD and the light pipes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a perspective front view of a display device inaccordance with one embodiment of the present invention having aremovable backlighting assembly partially inserted.

FIG. 1B illustrates a perspective back view of a display device inaccordance with one embodiment of the present invention having aremovable backlighting assembly removed.

FIG. 1C illustrates a front view of a desk top display device inaccordance with an embodiment of the present invention having a fixed inplace backlighting assembly or module.

FIG. 1D illustrates a back perspective view of the desk top displaydevice of FIG. 1C.

FIG. 2A is a cross sectional view of a dual light source and dual lightpipe embodiment of an LCD flat panel display in accordance with thepresent invention.

FIG. 2B is a cross sectional view of another implementation of the duallight source and dual light pipe embodiment of FIG. 2A.

FIG. 3A illustrates an extraction pattern disposed on the bottom side ofa light pipe in accordance with embodiments of the present inventionthat use a single edge-disposed light source per light pipe.

FIG. 3B illustrates a portion of the LCD panel embodiment of FIG. 2Awith oriented extraction patterns in accordance with the presentinvention.

FIG. 3C illustrates a portion of the LCD panel embodiment of FIG. 2Bwith oriented extraction patterns in accordance with the presentinvention.

FIG. 3D illustrates variation of the embodiment of FIG. 3C having twovariable intensity light sources and a single light pipe for both.

FIG. 4 is a schematic diagram of the inverter circuitry used toindependently control the brightness of light sources within an LCD flatpanel display within the embodiments of the present invention.

FIG. 5 illustrates the CIE chromaticity diagram including the black bodycurve from blue to red.

FIG. 6 is a graph illustrating the color temperatures achieved by oneimplementation of the dual light source and dual light pipe embodimentof the present invention for a given color temperature range.

FIG. 7A, FIG. 7B and FIG. 7C are spectrum graphs of the energydistributions over a range of wavelengths representing the colortemperature distributions of three exemplary blue light sources selectedin accordance with the present invention.

FIG. 8A and FIG. 8B are spectrum graphs of the energy distributions overa range of wavelengths representing the color temperature distributionsof two exemplary red light sources selected in accordances with thepresent invention.

FIG. 9A is a graph of color temperature and luminance for oneimplementation of the dual light source and dual light pipe embodimentof the present invention having a blue source at 11,670 K and a redsource at 3,623 K.

FIG. 9B is a graph of color temperature versus luminance for oneimplementation of the dual light source and dual light pipe embodimentof the present invention for a blue source at 15,599 K and a red sourceat 3,221 K with 2.6 mm cold cathode fluorescent tubes (CCFL).

FIG. 9C is a graph of color temperature versus luminance for oneimplementation of the dual light source and dual light pipe embodimentof the present invention for a blue source at 15,599 K and a red sourceat 3221 K with 2.4 mm CCFL.

FIG. 9D is a graph of color temperature versus luminance for oneimplementation of the dual light source and dual light pipe embodimentof the present invention for a blue source at 15,005 K and a red sourceat 3,561 K with 2.6 mm CCFL.

FIG. 10A is a cross sectional diagram of an embodiment of the presentinvention having dual light pipes and four light sources, two bluesources and two red sources.

FIG. 10B is a cross sectional diagram of an embodiment of the presentinvention having a single light pipe and four light sources, two bluesources and two red sources.

FIG. 11 is a cross sectional diagram of an embodiment of the presentinvention having dual light pipes and three light sources, two bluesources associated with one light pipe and one red light source that isadjusted for color balancing.

FIG. 12 is a cross sectional diagram of an embodiment of the presentinvention having two wedge-shaped cross-nested light pipes and two lightsources.

FIG. 13A illustrates a cross sectional diagram of another embodiment ofthe present invention in which backlight is recycled to increaseluminance of the LCD.

FIG. 13B illustrates a cross section of another embodiment of a rearreflector where the reflection material is applied to a plasticsubstrate or carrier.

FIG. 13C illustrates a cross section of a PTFF film reflector accordingto one embodiment of the present invention.

FIG. 14 illustrates an exemplary computer system in which the process ofselecting approprate light sources according to one embodiment of thepresent invention.

FIG. 15 is a flow diagram illustrating the process of selectingappropriate light source candidates according to one selection criterionin furtherance of one embodiment of the present invention.

FIG. 16 is a flow diagram illustrating the process of selectingappropriate light source candidates according to another selectioncriterion in furtherance of one embodiment of the present invention.

FIG. 17 is a flow diagram illustrating a process of selectingappropriate light source candidates according to yet another selectioncriterion in furtherance of one embodiment of the present invention.

FIG. 18A illustrates a backlighting embodiment of the present inventionhaving an array of CCF light sources.

FIG. 18B illustrates a backlighting embodiment of the present inventionhaving an array of CCF light sources and a scallop-shaped rearreflector.

FIG. 18C illustrates a backlighting embodiment of the present inventionhaving an array of CCF light sources and a scallop-shaped rear reflectorwhere each scallop has a light source pair.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the present invention, a colorbalancing system for a flat panel LCD unit applying variable brightnessto multiple light sources of varying color temperature, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone skilled in the art that the present invention may be practicedwithout these specific details or with certain alternative equivalentcircuits and methods to those described herein. In other instances, wellknown methods, procedures, components, and circuits have not beendescribed in detail as not to unnecessarily obscure aspects of thepresent invention.

Exemplary Display Systems

FIG. 1A and FIG. 1B illustrate front and back perspective views of adisplay system 5 in which embodiments of the present invention can beimplemented. Although exemplary display system 5 utilizes a removablebacklighting unit 14 that makes use of edge lighting technology, thecolor temperature balancing embodiments of the present invention areequally applicable to display systems that use fixed-in-place edge litbacklighting units and/or direct backlighting technology (FIG. 1C andFIG. 1D). The color temperature balancing embodiments of the presentinvention are equally applicable to direct backlighting applications aswell as edge lighting applications. As described more fully in U.S. Pat.No. 5,696,529, issued Dec. 9, 1997, by Evanicky et al., and U.S. Pat.No. 5.593,221, issued Jan. 14, 1997, by Evanicky et al, both of whichare assigned to the assignee of the present invention and incorporatedherein by reference, the high resolution color flat panel display 5 hasa backlighting door assembly (“backlighting assembly”)14 for directviewing. This backlighting assembly 14 can be removed to expose thetransparent active LCD unit screen. Once removed, the transparent activeLCD unit screen can be positioned on top of an overhead projector inorder to project the displayed image in an enlarged fashion onto areceiving screen.

With reference to FIG. 1A, a perspective view of the display subsystem 5is illustrated with the display side facing toward the viewer. This isthe direct viewing configuration. The display system 5 comprises threemajor assemblies. The base assembly 12 which is coupled to a displayassembly 10 via a hinge in order to allow the display assembly 10 toadjust to different angles for direct monitoring or allows the displayassembly to lay flat for overhead projection configurations and forstorage and transportation. The base assembly 12 supports the display 10for direct viewing configurations and also contains several electroniccircuit systems for providing the display unit with power, audioinformation, and video information (see FIG. 4). Herein, the doorassembly 14 is also called a backlighting assembly or a liquid crystalflat panel layer.

The display assembly 10 contains two stereo speakers 8 a and 8 b as wellas an active matrix LCD color screen 20. Although many differentresolutions can be utilized within the scope of the present invention,an implementation utilizes an LCD screen 20 having 1280 pixels by 1024pixels by RGB color and utilizes amorphous silicon thin film transistors(TFT). The LCD screen 20 is composed of color TFT-LCD panel, driver ICs,control circuitry, and power supply circuitry all contained in a rigidbezel. LCD screen 20 is capable of displaying 2¹⁸ true colors withoutframe rate modulation in text or graphics mode. Various flat panel LCDscreens and screen technologies can be used within the scope of thepresent invention with proper configuration.

As shown in FIG. 1A, the display assembly 10 is back-lit via a separateassembly or removable backlighting assembly 14. In this view, the dooris partially removed from the display assembly 10. The backlightingassembly 14 is removed so that the display 20 can become transparent foroverhead projection configurations. While inserted, the backlightingassembly 14 provides backlighting for the LCD screen 20 for directviewing configurations. Although a number of lamps can be utilized, oneembodiment utilizes cold cathode fluorescent (CCF) tubes which arelocated within the display assembly to illuminate along the top andbottom edges of one or more light pipes located within the backlightingassembly 14 (as will be discussed further below) when the backlightingassembly 14 is inserted within the display assembly 10. Hot cathodetubes (HCF) can also be used. Also shown is a snap fit clip 34 which isused to secure the backlighting assembly 14 to the display assembly 10.

FIG. 1B illustrates the back side of the display subsystem 5 with thebacklighting assembly 14 completely removed to expose inner componentsof the display assembly 10. In this view, with the backlighting assembly14 removed, the back side of the LCD screen 20 is exposed. Located onthe base assembly 12 are inputs for AC power 44 and an audio/video inputconnector 48. Power supplied to the subsystem, backlight brightness andaudio volume are controlled by the computer system's software throughthe audio/video input connector 48. In an alternative embodiment, inaddition to computer control these features can be manually adjusted.For instance, also located on the display subsystem 5 can be(optionally) a power on switch 2 a, a color temperature adjustment knob2 b and a volume adjustment knob 2 c for the stereo speakers 8 a and 8b. The audio/video input connector 48 is coupled to the digitalaudio/video output of a computer system.

Located within the display assembly 10 are two lamp assemblies orhousings. One lamp housing 40 is shown. Each lamp housing can containone or more CCF lamps 52, depending on the particular embodiment ofwhite balancing utilized (described further below). The CCF lamps canoptionally be mounted within their respective lamp housing using tworubber shock mounts, as shown, 50 a and 50 b to secure lamps 52. Anidentical configuration is employed for the top lamp housing (obscured).A reflective film 42 is applied to the inner portions of the lamphousings and this tape extends outside, beyond the positions of thelamps 52, for providing an optical coupling with components of thebacklighting assembly 14 when inserted. The same is true for the upperlamp housing.

Also shown in FIG. 1B are two receiving holes 32 located on the rightand left sides of the display assembly 10. These receiving holes 32fasten to corresponding latches (34 not shown) located on thebacklighting assembly 14. There is also a recess associated with theselatch holes 32 for removal of the backlighting assembly 14. Also locatedwithin this region of the display assembly 10 is a magnetic reed switch22 that is responsive to the presence of a magnet 140 (not shown) thatis located along the mating edge of the backlighting assembly 14. Usingthis switch 22, the display subsystem 5 determines whether or not thebacklighting assembly 14 is inserted or removed from the displayassembly 10 and responds accordingly. It is appreciated that the reedswitch 22 and sensor, in lieu of being magnetically operated, can alsobe implemented using and optical sensor (or switch, such as using a LEDor fiber-optic device) or a mechanical sensor (or switch, such as atoggle or spring switch).

There are also two notches 95 a and 95 b located on the top of thedisplay assembly 10. These notches 95 a and 95 b are for mating withcorresponding latches located on an overhead projector of the presentinvention for securing the display subsystem properly over anilluminating screen of the projector. When used in a projectorconfiguration, the display subsystem 5 is extended so that the baseassembly 12 and the display assembly 10 are flat and the facing side ofthe display subsystem, as shown in FIG. 1B, is placed facing down on topof the illuminating screen of the projector. In this way, light isprojected through the back side of the LCD screen 20.

As discussed above, the color temperature balancing embodiments of thepresent invention are equally applicable to display systems that usefixed-in-place edge lit backlighting units and/or direct backlightingtechnology. FIG. 1C illustrates a front view of a desk top display unit(“monitor”) 36 having installed therein an LCD flat panel displayassembly 38 having the color balancing system of the present invention.In- this embodiment, the LCD flat panel display assembly 38 isfixed-in-place and edge lit with light sources along the top horizontaledge 38 a and bottom horizontal edge 38 b.

FIG. 1D illustrates a back perspective view of the desk top display unit36. This desk top display unit 36 includes a mounting bracket 39 formounting on a wall, a mechanical arm or for mounting with a base.

Color Balancing Systems of the Present Invention

FIG. 2A illustrates a liquid crystal flat panel display (herein “flatpanel display”) 110 a in accordance with one embodiment of the presentinvention. This flat panel display 110 a can be used within a flat paneldisplay device having a fixed-in-place backlighting unit or can be usedwithin a flat panel display system 5 (FIG. 1A) using a removablebacklighting assembly 14. The flat panel display 110 a, in accordancewith the present invention, provides white balance adjustment byindependently varying the brightness of a pair of light sources (e.g.,CCF tubes) 132 and 136. For a predetermined range of color temperatures,having a minimum temperature (e.g., 5,000 K) and a maximum temperature(e.g., 6,500 K), a first light source 132 is provided that has awavelength spectrum with an overall color temperature less than theminimum temperature of the predetermined range; herein, a light source132 with this characteristic is called the “red” light source forconvenience. Also, a second light source 136 is provided that has awavelength spectrum with an overall color temperature that is greaterthan the maximum temperature of the predetermined range; herein, a lightsource 136 with this characteristic is called the “blue” light sourcefor convenience.

As shown in FIG. 2A, the red light source 132 is optically coupled toprovide light to a first planar light pipe 130. The red light source 132is positioned along an edge of the light pipe 130. In FIG. 2A, onlycross sections of this planar light pipe 130 and the light source 132are shown. Likewise, the blue light source 136 is optically coupled toprovide light to a second planar light pipe 134. The blue light source136 is positioned along an edge of the light pipe 134. Only crosssections of this planar light pipe 134 and the light source 136 areshown. In the embodiment 110 a of FIG. 2A, the light sources 132 and 136are long thin tubes which are positioned on opposite sides of the planarlight pipes 134 and 130. The light sources 132 and 136 are positioned tobe substantially parallel with each other. The power supply for eachlight source 132 and 136 receives a separate voltage signal forindependently controlling its brightness with respect to the other lightsource. It is appreciated that the positions of the red tube 132 and theblue tube 136 can be switched without departing from the scope of theinvention.

In order to maintain independence of the light distribution between thefirst 130 and second 134 light pipes, an air gap 133 is maintainedbetween the two light pipes. This air gap 133 prevents light from lightsource 132 being extracted within light pipe 134 and prevents light fromlight source 136 from being extracted within light pipe 130. The air gap133 is particularly important for embodiment 110 a because each lightpipe is illuminated from one edge, and the extraction dot pattern (e.g.,a pattern of bumps disposed on the surface of the light pipes)corresponding to that light pipe is specifically tailored for lightoriginating from that edge. On the edges and surrounding the light pipesare reflection tapes 131 and 135.

Within embodiment 110 a, a rear reflector layer 138 is positioned on oneside of the light pipes. On the other side of the light pipes a diffuserlayer 128 (mylar) is followed by one or more brightness enhancementlayers (BEFs) 126, followed by a double BEF (DBEF) layer 124 which isfollowed by a cover layer 122 for protection. The DBEF layer 124redirects light not of the proper polarization to the rear reflectorlayer 138 for recycling. The LCD panel includes a first polarizer layer120 followed by a back glass layer 121 followed by the selectivelyenergized transistor layer 119 and an LCD layer 118, followed byred/green/blue color filter layers 114, a front glass layer 115 followedby a second polarizer layer 116. A glass or acrylic protection layer 112is then used. These layers are described in more detail further below.

The white balance or color temperature of the embodiment 110 a ismaintained and adjusted using the two independently controlled lightsources 132 and 136. The white balance is adjusted by altering thebrightness of the light sources 132 and 136 independently. The phosphormix (e.g., contribution of red, green and blue phosphor) of the twolight sources 132 and 136 is selected so that the white balance can beadjusted by varying the brightness of the light sources. The light pipes130 and 134 are acrylic and each contain an extraction system thatuniformly and independently distributes the light from each light sourceacross the viewing area of the display.

In one implementation, the light pipes 130 and 134 are mounted toaddress a removable backlight assembly (e.g., assembly 14). In anotherimplementation, the light sources are located behind a diffusing system128 to directly backlight the display rather than “edge” light the lightpipe. In one embodiment, the light sources 132 and 136 are cold cathodefluorescent (CCF) tubes and, in another embodiment, hot cathodefluorescent (HCF) tubes are used. Constraints are placed on the amountof brightness variation tolerated during white adjustment such that theoverall brightness of the display never decreases below a percentage ofthe maximum brightness output by the light sources 132 and 136. In oneimplementation, this percentage is selected at 70 percent.

FIG. 2B illustrates another embodiment 110 b that is analogous toembodiment 110 a except for the differences pointed out below. In theembodiment 110 b of FIG. 2B, the light sources 132 and 136 are long thintubes which are positioned on the same side of the planar light pipes134 and 130. The light sources 132 and 136 are positioned to besubstantially parallel with each other. Because the light sources 132and 136 are positioned on the same side of the planar light pipes 134and 130, the position of the reflection element 131 is shifted. It isappreciated that the positions of the red tube 132 and the blue tube 136can be switched without departing from the scope of the invention.

Extraction Pattern Orientation

FIG. 3A illustrates a top view of an exemplary extraction pattern 144 athat can be applied to the bottom of light pipe 130 within embodiment110 a. The extraction pattern 144 a is designed to uniformly illuminatethe LCD layer 118, at any brightness, taking into consideration that thered tube 132 is positioned along one edge of the light pipe 130. Toaccomplish this uniform distribution of light, extraction dots increasein size in a proportion to their distance from the light source 132 asshown in direction 146. Extraction dots 150 a are smaller since they arerelatively close to the light source 132. Extraction dots 150 b areslightly larger since they are relatively farther from to the lightsource 132 than dots 150 a. Extraction dots 150 c are the largestbecause they are the farthest from light source 132. It is appreciatedthat extraction pattern 144 a also includes larger sized dots 150 d atthe comers near the light source 132 because the tube 132 is not asbright at the ends as in the middle sections of the tube.

FIG. 3B illustrates a configuration 160 of light pipes and light sources(of embodiment 110 a of FIG. 2A) taking into consideration theorientation of the light extraction patterns. Within embodiments 110 aand 110 b, each light extraction pattern is designed to operate with itsown light pipe (e.g., pipe 130) independently of the other light pipe(e.g., pipe 134). In other words, extraction pattern 144 a is designedto uniformly distribute light to the LCD layer 118, independently oflight pipe 132, as the brightness of light source 132 varies. Extractionpattern 144 b is designed to uniformly distribute light to the LCD layer118, independently of light pipe 130, as the brightness of light source136 varies. Light extraction pattern 144 a is shown in FIG. 3B in crosssection as a thin line applied to the underside of light pipe 130. Asshown, the dot sizes increase within pattern 144 a from left to rightbecause the light source 132 is positioned on the left edge of the lightpipe 130. However, the light extraction pattern 144 b applied to theunderside of light pipe 134 is the flipped image of pattern 144 a withthe dot sizes increasing from right to left because light source 136 ispositioned along the right edge of the light pipe 134.

Considering the provision of the air gap 133 and that each lightextraction pattern 144 a and 114 b is tailored for its own light pipe,the light pipes 130 and 134 effectively operate separately andindependently to uniformly distribute light over the LCD layer. Onefunction of the light extraction patterns 144 a and 144 b is touniformly distribute light over their associated light pipes even if onelamp is dimmed (or brightened) unilaterally. It is appreciated that thebrightness of light source 136 is increased slightly to compensate forthe fact that extraction pattern 144 a resides between the light pipe134 and any LCD layer and thereby slightly obstructs the light emittedfrom light pipe 134. An alternative approach adjusts the sizes of thedots of the relative dot extraction patterns to compensate for theobstruction.

FIG. 3C illustrates a configuration 165 of light pipes and light sources(of embodiment 110 b of FIG. 2B) taking into consideration theorientation of the light extraction patterns. Within embodiments 110 aand 110 b, each light extraction pattern is designed to operate withtheir light pipe (e.g., pipe 130) independently of the other light pipe(e.g., pipe 134). Light extraction pattern 144 b is shown in FIG. 3C incross section as a thin line applied to the underside of light pipe 130.As shown, the dot sizes increase within pattern 144 b from right to leftbecause the light source 132 is positioned on the right edge of thelight pipe 130. The same light extraction pattern, 144 b, is alsoapplied to the underside of light pipe 134. As discussed above,extraction pattern 144 b is the mirror image of pattern 144 a with thedot sizes increasing from right to left because light source 136 ispositioned along the right edge of the light pipe 134.

FIG. 3D illustrates a variation of the embodiment 165 of FIG. 3C.Alternatively, as shown in FIG. 3D, this embodiment 167 uses bothcontrols for the first 132 and second 136 light sources together tochange the display brightness without altering the white balance settingwhere both the first and second light sources are positioned on the sameside of a single light pipe layer 130′ and optically coupled to it. Arear reflector 138 is also used. This embodiment 167 can also be usedfor color temperature balancing.

Dual Inverters for Independently Driving Light Sources

FIG. 4 is a logical block diagram of electronics 170 of the displaysubsystem 5. Although some electrical components are shown (in dashedlines) to be associated with the base assembly 12 or the displayassembly 10, it is appreciated that their locations are exemplary. Apartfrom the LCD screen 20, the actual location of the circuits could be ineither the display assembly 10 or the base assembly 12. It isappreciated that circuit 170 includes the provision of separate invertercircuits 175 a and 175 b for separately and independently controllinglight sources 132 and 136. A color temperature adjustment knob 2 b (FIG.1B) is coupled to circuit 187 which controls the voltages supplied bythe inverters 175 a and 175 b to the lamps 132 and 136 over bus 180 toseparately control their brightness. Another implementation adjusts thebrightness through software control by means of a digital potentiometer.Inverter 175 a controls light source 132 and inverter 175 b controlslight source 136.

In one implementation, within the base assembly, as shown in FIG. 4, area power supply unit 184 for coupling with an alternating current source44. This power supply 184 supplies power via line 182 to an audio board178 and a video board 176. The audio board 178 is coupled to the videoboard 176 via bus 186. Audio and video information are sent to thedisplay subsystem via input interconnect 48. It is appreciated that avariety of audio/video information transfer formats and standards can beused within the scope of the present invention, including an IBMcompatible standard, a UNIX standard, or Apple Computer standard.

Video board 176 is coupled to a bus 185 for communicating andcontrolling elements of the display assembly 20. It is appreciated thatportions of bus 185 are composed of flex circuits so that base assembly12 and display assembly 10 can move freely about their common hinge.Among other signals, this bus 185 carries power, control signals andaudio and video data signals. The video board 176 is coupled to supplyaudio signals over bus 185 to stereo speakers 8 a and 8 b. Video board176 also supplies a control signal and power over line 185 to a circuit187 which in turn independently controls two AC to DC inverters 175 aand 175 b. Each inverter contains a transformer for supplying a highvoltage signal, over bus 180, to the light sources 132 and 136 and alsocontains a switch circuit for turning the tubes off. Light sources 132and 136 are separately coupled to power supply lines 180 a and 180 b,respectively, which are within bus 180. Return bus 189 contains aseparate return lines from source 132 to inverter 175 a and from source136 to inverter 175 b. Bus 185 is also coupled to reed switch 22 whichcarries a digital signal indicating when the backlighting assembly 14 isinserted into the display assembly 10 or not present.

Bus 185 of FIG. 4 is coupled to supply video information to columndriver circuits 171. The column driver circuits 171 control informationflow to the columns of each of the rows of transistors of the LCD screen20 to generate an image in the well-known fashion. (There are alsoseparate row driver circuits that are not illustrated but operate in thewell-known fashion.)

Blackbody Chromaticity Curve

FIG. 5 illustrates a CIE chromaticity diagram illustrating chromaticitycoordinates along the horizontal and vertical. Within the diagram 190,the green portion 194 is toward the top with yellow 192 between green194 and red 198. Blue 196 is toward the left. A black body curve 200represents the chromaticity displayed by a tungsten filament heated tovarious degrees kelvin. For instance, from point D to point A alongcurve 200, the curve represents the color emitted from the tungstenfilament from 6,500 degrees K to 2856 degrees K. As shown, the blackbodycurve 200 traverses from blue 196 to the red 198 without straying muchinto the yellow 192 or green 194 regions.

The light sources 132 and 136 selected in accordance with the presentinvention are those that illuminate with a color temperature that isnear the blackbody curve 200 when their brightness is adjusted within apredetermined color temperature range (e.g., 5,000 to 6,500 K). That is,the color balancing system of the present invention allows adjustmentsto the color temperature of the flat panel display screen that remainclose to the blackbody curve.

In addition to following the CIE black body curve (“locus”), lightsources 132 and 136 selected in accordance with the present inventionfollow the daylight color temperature locus when color temperature ofthe display is adjusted. One advantage of following the daylight colortemperature locus during white balancing is that the resulting colortemperature tends to be brighter (e.g. having a greater lumen value, Y),and tends to model daylight more accurately. Furthermore, the resultingcolor temperature tends to be more “green,” giving the display a morenatural appeal.

FIG. 6 illustrates one exemplary case where the “blue” light source 136is a CCFL tube having a color temperature of 11,670 K and the “red”light source 132 is a CCFL tube having a color temperature of 3,623 Kwithin a flat panel display having a color balancing system 160 as shownin FIG. 3B. FIG. 6 illustrates that by independently varying thebrightness of the light sources 132 and 136, the resultant colortemperature of the flat panel display can be altered in accordance withline 212. Line 200 is the same blackbody curve as shown in FIG. 5. Inthis case, curve 212 is substantially similar to curve 200 within thepredetermined color temperature range of 5,000 K to 6,500 K. In oneembodiment, the brightness is varied by holding one light source (e.g.,132) at maximum brightness and dimming the other light source (e.g.,136) until a minimum brightness threshold is met. This steers the colortemperature from a mid range value (e.g., 5500 K) into progressivelywarmer (e.g. smaller) values. To increase the color temperature from themid range value, light source 136 is held constant and light source 132is dimmed down until the minimum threshold brightness is reached.

Example Red and Blue Light Sources

Within the present invention, the light sources 132 and 136 are selectedsuch that their color temperature allows the white balancing within apredetermined range (e.g., 5,000 to 6,500) that (1) follows theblackbody curve 200, (2) where the overall brightness of the displaydoes not drop below a predetermined threshold over the color temperaturerange and (3) having a peak brightness (both light sources on) near themiddle of the color temperature range. There are many combinations ofblue and red tubes that meet the above constraints. Processes describedfurther below illustrate the manner in which light sources 132 and 136can be selected that meet the above constraints. The followingdescription illustrates some exemplary blue light sources and exemplaryred light sources that can be paired (in blue-red combinations) to meetthe above constraints.

FIG. 7A illustrates a blue light source 136 having percentages of red,green and blue phosphors such that the CCFL tube exhibits the emissionspectrum 220 within the 375-775 nm wavelength range. The overall colortemperature for the light source of FIG. 7A is very high at 15,600 K.FIG. 7B illustrates a blue light source 136 having percentages of red,green and blue phosphors such that the CCFL tube exhibits the spectrum224 within the 375-775 nm wavelength range. The overall colortemperature for the light source of FIG. 7B is high at 15,000 K. FIG. 7Cillustrates a blue light source 136 having percentages of red, green andblue phosphors such that the CCFL tube exhibits the spectrum 226 withinthe 375-775 nm wavelength range. The overall color temperature for thelight source of FIG. 7C is 10,600 K.

FIG. 8A illustrates a red light source 132 having percentages of red,green and blue phosphors such that the CCFL tube exhibits the spectrum230 within the 375-775 nm wavelength range. The overall colortemperature for the light source of FIG. 8A is low at 3,560 K. FIG. 8Billustrates a red light source 132 having percentages of red, green andblue phosphors such that the CCFL tube exhibits the spectrum 232 withinthe 375-775 nm wavelength range. The overall color temperature for thelight source of FIG. 8B is low at 3,220 K.

Exemplary combinations of the above red and blue light sources that canbe used within embodiment 160 of FIG. 3B are shown in Table I below.

TABLE I Pair Red Tube Blue Tube Ctemp @ Max Lumin 1 3,560 15,600 5,893 23,220 15,600 5,635 3 3,560 10,600 5,468 4 3,560 15,000 5,635 5 3,22015,000 5,367 Where Ctemp @ Max Lumin is the color temperature of thepair at maximum luminance.

FIG. 9A illustrates a color temperature and luminance diagram 240 forvarious brightness configurations of a color balancing embodiment 160 ofFIG. 3B within the color temperature range of 5,000 to 6,500 K. In thisexample, a blue tube 136 having a color temperature of 11,670 K is usedwith a red tube 132 having a color temperature of 3,623 K andcorresponds to the same configuration described with respect to FIG. 6.Mid point 246 c of FIG. 9A represents maximum luminance when both tubesare at their full brightness and a color temperature near 5,500 K isreached. This is roughly in the middle of the color temperature range5,000 to 6,500 K.

The following describes color temperature variations from the mid point246 c achieved by dimming one or the other tube. Region 246 a representsthe white balance adjustment where the red tube 132 is left fully on andthe blue tube 136 is dimmed down in a range from 5 to 25 percent (of theoriginal full) luminance. Within region 246 a, curve 242 represents theluminance ratio and this value decreases (from 1.0 to 0.8) as the bluetube 136 is dimmed down. Also within region 246 a, the color temperatureas shown by curve 248 decreases as the blue tube 136 is dimmed down.Region 246 b represents the white balance adjustment where the blue tube136 is left fully on and the red tube 132 is dimmed down from 5 to 25percent of the original full luminance. Within region 246 b, curve 242represents the luminance ratio and this value decreases (from 1.0 to0.8) as the red tube 132 is dimmed down. Also within region 246 b, thecolor temperature as shown by curve 248 increases as the red tube 132 isdimmed down.

FIG. 9B illustrates a color temperature and luminance diagram 260 forvarious brightness configurations of a color balancing embodiment 160 ofFIG. 3B within the color temperature range of 3,400 to 8,250 K usingCCFL tubes of 2.6 mm in size. In this example, a blue tube 136 having acolor temperature of 15,599 K (FIG. 7A) is used with a red tube 132having a color temperature of 3,221 K (FIG. 8B). Curve 262 representsthe luminance in Cd/sq m over the given range of color temperatures andcurve 264 represents the luminance ratio (from 0 to 1.0). Peak luminancepoint 266 represents the maximum brightness condition (4,600 K) whenboth lamps 136 and 132 are fully on. That portion of the curves to theright of point 266 represents the condition when tube 136 is fully onand tube 132 is dimmed down to increase the color temperature. Thatportion of the curves to the left of point 266 represents the conditionwhen tube 132 is fully on and tube 136 is dimmed down to decrease thecolor temperature.

FIG. 9C illustrates a color temperature and luminance diagram 270 forvarious brightness configurations of a color balancing embodiment 160 ofFIG. 3B within the color temperature range of 3,400 to 8,250 K usingCCFL tubes of 2.4 mm in size. In this example, a blue tube 136 having acolor temperature of 15,599 K (FIG. 7A) is used with a red tube 132having a color temperature of 3,221 K (FIG. 8B). Curve 272 representsthe luminance in Cd/sq m over the given range of color temperatures.Peak luminance point 276 represents the maximum brightness condition(5,000 K) when both lamps 136 and 132 are fully on. That portion of thecurves to the right of point 276 represents the condition when tube 136is fully on and tube 132 is dimmed down to increase the colortemperature. That portion of the curves to the left of point 276represents the condition when tube 132 is fully on and tube 136 isdimmed down to decrease the color temperature.

FIG. 9D illustrates a color temperature and luminance diagram 280 forvarious brightness configurations of a color balancing embodiment 160 ofFIG. 3B within the color temperature range of 3,400 to 8,250 K usingCCFL tubes of 2.6 mm in size. In this example, a blue tube 136 having acolor temperature of 15,599 K (FIG. 7A) is used with a red tube 132having a color temperature of 3,221 K (FIG. 8B). Curve 282 representsthe luminance in Cd/sq m over the given range of color temperatures.Peak luminance point 286 represents the maximum brightness condition(4,800 K) when both lamps 136 and 132 are fully on. That portion of thecurves to the right of point 286 represents the condition when tube 136is fully on and tube 132 is dimmed down to increase the colortemperature. That portion of the curves to the left of point 286represents the condition when tube 132 is fully on and tube 136 isdimmed down to decrease the color temperature.

It is appreciated that many of the layers within an LCD flat paneldisplay system tend to “yellow” shift light passing there through, e.g.,the acrylic in the light pipes, the ultra-violet cured extractionpatterns, the DBEF and BEF films, the polarizers, the LCD layer and thecolor filters. Therefore, to compensate for this yellow shift, the redand/or blue light sources 132 and 136 selected can be slightly blueshifted.

Additional Multi-Light Source Embodiments

FIG. 10A illustrates a cross section of an alternate embodiment 310 of acolor balancing system in accordance with the present invention thatutilizes four light sources. Two red light sources 312 and 314 arepositioned on opposite sides of a planar light pipe 130 and are parallelwith each other. The brightness of these two red light sources 312 and314 are varied in tandem. Two blue light sources 316 and 318 arepositioned on opposite sides of a planar light pipe 134 and are parallelwith each other. The brightness of these two blue light sources 316 and318 are varied in tandem independently of the red light sources 312 and314. It is appreciated that the positions of the blue and red lightsources can be switched in accordance with the present invention. Lightpipe 134 is positioned under light pipe 130. An air gap 133 ispositioned between the light pipes 130 and 134 but is optional in thisembodiment because the locations of the red and blue light source pairsare symmetrical with respect to both light pipes 130 and 134. As withembodiment 160, CCFL tubes or HCL tubes can be used as the light sourceswith particular red, green, blue phosphor contributions to differentiatethe blue from the red light sources.

Within embodiment 310, because the brightness of the light sources thatare on opposite sides of a same light pipe are varied in tandem, thereis a uniform decrease or increase in brightness on both sides of thelight pipe (e.g., light pipe 130 or 134). In this case, the extractionpattern 144 c applied to the underside of each light pipe 130 and 134utilizes extraction dots that vary in size with respect to their closestdistance from the two light sources. That, is, along the sides havingthe light sources, the extraction dots are smaller and they increase insize (from both sides) toward the middle. An extraction pattern 144 cfitting this description is described in U.S. Pat. No. 5,696,529, issuedDec. 9, 1997 by Evanicky, et al., and assigned to the assignee of thepresent invention.

In accordance with the embodiment 310 of FIG. 10A, to vary the colortemperature of the display, the voltage driving the red light sources312 and 314 is varied to vary their brightness. With the blue lightsources 316 and 318 at maximum brightness, the color temperature can beincreased from mid-range by dimming down the red light sources 312 and314 in tandem. Conversely, to vary the color temperature of the display,the voltage of the inverter supply driving the blue light sources 316and 318 is varied to vary their brightness. With the red light sources312 and 314 at maximum brightness, the color temperature can bedecreased from mid-range by dimming down the blue light sources 316 and318 in tandem.

Embodiment 310 provides increased brightness through the colortemperature variation because more light sources are utilized.Therefore, this embodiment 310 has a larger pool of red/blue lightsource candidates which allow good color temperature range variationwhile also providing adequate brightness through the color temperaturerange. However, since more light sources are used in embodiment 310 overthe dual light pipe embodiment 160, embodiment 310 consumes more power.

FIG. 10B illustrates a cross section of an embodiment 330 of a colorbalancing system in accordance with the present invention that is avariation of embodiment 310. Embodiment 330 includes a single planarlight pipe 340 having a red/blue pair of light sources located on twoopposite sides. On the left are located a red light source 312 and ablue light source 316 and on the right are located a red light source314 and a blue light source 318. Like embodiment 310, the red lightsources 312 and 314 of embodiment 330 are varied in tandem and the bluelight sources 316 and 318 are varied in tandem, independently from thered light sources 312 and 214. An extraction pattern 144 c, as describedabove, is applied to the underside of light pipe 340.

Color temperature variation is performed for embodiment 330 in the samemanner as described with respect to embodiment 310. The advantage ofembodiment 330 is that a single light pipe 340 can be used. Since thebrightness of the red and blue light sources are varied in tandem (for agiven color), only two inverters 175 a and 175 b (FIG. 4) are requiredfor embodiment 310 and for embodiment 330. In other words, the voltagesignal on line 180 a of FIG. 4 can be coupled to control both red lightsources 312 and 314 and the voltage signal on line 180 b can be coupledto control both blue light sources 316 and 318.

FIG. 11 illustrates a cross section of another embodiment 350 of a colorbalancing system in accordance with the present invention that utilizestwo blue light sources 316 and 318 and a single red light source 314.The blue light sources 316 and 318 are positioned along opposite edgesof a first light pipe 130. An extraction pattern 144 c, as describedabove, is applied to the underside of light pipe 130. Positioned underlight pipe 130 (with an air gap 133 in between) a second light pipe 134.A single red light source 314 is positioned along one edge of light pipe134 (e.g., on the right or left side). When light source 314 ispositioned on the right side, as shown, extraction pattern 144 b is usedwith light pipe 134 and when light source 314 is positioned on the leftside, extraction pattern 144 a is used.

In operation, the blue light sources 316 and 318 are maintained at (orslightly above) a color temperature above a predetermined colortemperature range (e.g., at or above 6,500 K). The blue light sources316 and 318 are maintained at or near their full brightness to providethe required luminance for the display and the red light source 314 isadjusted in brightness to provide a varying degree of down-shifted colortemperature. Embodiment 350 provides the advantage that the colortemperature of the display can effectively be adjusted without affectingthe backlight luminance. That is to say, if the emission spectra of thered lamp is in the deep red region (e.g., 658 nm), then even at its fullbrightness it would only contribute about 5 percent of the backlightluminance because of the human eye's insensitivity to that color region.It is appreciated that two inverters 175 a and 175 b are required forembodiment 350 even though the brightness of light sources 316 and 318is held constant. Power consumption for the red light source 314 iswithin the region of 0.5 watt.

FIG. 12 illustrates a cross section of an embodiment 370 of a colorbalancing system in accordance with the present invention. Embodiment370 is similar to embodiment 160 except the planar light tubes 372 and374 are wedge-shaped in cross section. The light tubes 372 and 374 arepositioned as shown in FIG. 12 so that they have a lower profile incross section. That is, the light pipes 130 and 134 of embodiment 160,in one implementation, are roughly 3 mm thick so their total width isjust over 6 mm when stacked with an air gap 133 in between. However,because the wedge-shaped light pipes 372 and 374 can be positioned asshown in FIG. 12, the overall height of light pipes 372 and 374 is only3 mm since they are inter-crossed (e.g., cross-nested) together. Asshown, a modification of extraction pattern 144 a is applied to theunderside of light pipe 372 and a modification of extraction pattern 144b is applied to the underside of light pipe 374. The functionality ofembodiment 370 similar to embodiment 160 however embodiment 370 offers amuch lower profile and reduced weight. A modification of the extractionpattern is necessary to compensate for the influence on extractionresulting from the angle of the wedge of each light pipe 372 374.

Color Balancing User Interface

There are several mechanisms in which the user can adjust the colorbalance of the display in accordance with the present invention. In oneembodiment, the user can adjust a slider between two extreme mechanicalpositions in which the position of the slider (or knob 2 b) represents aparticular color temperature within the predetermined color temperaturerange. The particular color temperature selected is then translated intoa dimming configuration by which one or more tubes are dimmed to achievethe color temperature.

In another embodiment, the slider is provided but the display alsocontains a chromaticity measuring device (e.g., a calorimeter) withgives the user immediate feedback as to the color temperature of thedisplay. The user then monitors the measuring device while adjusting theslider mechanism until the desired color temperature is reached.Alternatively, the white balance can be set via a feedback loop from acolorimeter positioned so that it analyzes the color temperature of thedisplay and feeds that information to the host computer through a serialport and the host computer then automatically adjusts the white balance.

Improved Light Pipe Assembly Construction for Enhanced DisplayBrightness

In prior art flat panel LCD systems, almost 50% of the luminance fromthe backlight is lost. This is due to the fact that the rear polarizerfilm of the LCD only accepts light with a specific orientation andrejects the rest. The present invention recognizes that, if thisrejected light could somehow be “recycled,” the net display brightnesscould be increased. FIG. 13A illustrates on embodiment of the presentinvention in which backlight is recycled to increase luminance of theLCD.

Particularly, as illustrated in FIG. 13A, portions of a flat panel LCD410 including a backlight distributor 430, which may comprise variousconfigurations of light pipes and light sources, such as the embodimentsof the dual light pipes illustrated in FIGS. 2A-2B and FIGS. 3B-3C. Inaddition, according to the present embodiment, flat panel LCD 410further includes a rear reflector layer 438 positioned on the back sideof the backlight distributor 430 for recycling light. On the other sideof the backlight distributor 430, a diffuser layer 428 is followed byone or more brightness enhancement layers (BEFS) 426, followed by a dualbrightness enhancement film (DBEF) layer 424 which is followed by aspecial cover layer 422. The LCD panel includes a first (rear) polarizerlayer 120 followed by a selectively energized transistor-layer 110 and aLCD layer 118, followed by red/green/blue color filter layers 114, afront glass layer 115 followed by a second polarizer layer 116. A glassor acrylic protection layer 112 is then used.

According to the present embodiment, DBEF layer 424 of FIG. 13A, incombination with the rear reflector 438, increases brightness of flatpanel LCD 410 by first “pre-polarizing” the light emitted from thebacklight distributor 430 to the same orientation as the rear polarizer120. Any light having the wrong orientation is reflected by DBEF layer424 to the rear reflector 438 where much of it is rephased and reflectedback to the display or lost through absorption. A large portion of therephased and reflected light from the reflector 438 can then passthrough the rear polarizer layer 120.

Naturally, the amount of light that can be recycled depends upon thereflectivity of rear reflector 438. Conventional reflective materialscurrently employed by the industry only reflect about 92% of the light.According to one embodiment of the present invention, a white rearreflector 438 made of a Teflon-like material developed by W.I. Gore andAssociates, Inc., and sold under the name of PTFF, is used. In thisembodiment, a near 100% increase in luminance is attainable. In oneembodiment, rear reflector 438 comprises a layer of PTFF 437 coated ontoplastic film substrate 441, and is illustrated in FIG. 13C. It should beapparent to those ordinarily skilled in the art, upon reading thepresent disclosure, that other materials displaying similarlight-rephasing and reflectivity properties may also be used.

LCD brightness may be further enhanced by the using a brightnessenhancement film (BEF) in between the backlight distributor 430 and theDBEF layer 424. As illustrated in FIG. 13A, flat panel LCD 410 includesoptional BEF layer(s) 426. In one embodiment where a single layer of BEFis used, LCD brightness may be increased by 50%. An even higherluminance gain may be attained when two layers of BEF are aligned at anangle of 90 to each other, a forming a “crossed BEF” layer. In somesystems, a luminance gain of 75% may be attained when crossed BEFs areused. An additional benefit of the crossed BEF is that the viewing angleof the LCD is significantly increased.

However, one drawback of the DBEF is that off-axis visual artifacts suchas color pattern or color stripes, may appear on the LCD screen ifviewed at large angles from the normal direction. In furtherance of oneembodiment of the present invention, a special cover sheet 422 may beused to eliminate such off-axis visual artifacts. According to thepresent embodiment, special cover sheet 422 comprises a thin lightdiffusing film placed between the DBEF and the rear polarizer of the LCDsuch that a small portion of the backlight is scattered at largeoff-axis angles. As a result, the contrast of the color stripe patternis greatly reduced. However, it should be noted that special cover sheet422 should not cause significant de-polarization because excessde-polarization would reduce the efficiency of the light-recyclingprocess. In the present embodiment, special cover sheet 422 is made ofan Lexan 8A35 material available from General Electric. Further, in thepresent embodiment, the special cover sheet 422 is cut with its opticalaxis matching the transmission axis of the polarizer and the DBEF.

In furtherance of the present invention, FIG. 13B illustrates anotherembodiment of a rear reflector 440 that may be used in place of rearreflector 438 of FIG. 13A. As shown, rear reflector 440 comprises alayer of barium sulfate 439 deposited on substrate 441 such as whiteplastic film(s). According to the present embodiment, barium sulfatelayer 439 may be deposited on substrate 441 by first mixing bariumsulfate powder with an organic binder to form a paste, and then screenprinting the paste on the substrate 441. Significantly, barium sulfatelayer 439 should be at least 0.01″ thick. It should be apparent to thoseof ordinary skill in the art, upon reading the present disclosure, thatnumerous well known organic binders and coating techniques may also beused to manufacture rear reflector 440.

Selecting Appropriate Percentage Composition of Phosphors for Use inCold Cathode Fluorescent Tubes

Portions of the present invention comprise computer-readable andcomputer-executable instructions which reside in, for example,computer-usable media of a computer system. FIG. 14 illustrates anexemplary computer system 500 upon which one embodiment of the presentinvention may be practiced. It is appreciated that system 500 of FIG. 14is exemplary only and that the present invention can operate within anumber of different computer systems and/or electronic device platforms.

System 500 of FIG. 14 includes an address/data-bus 510 for communicatinginformation and a central processor unit 520 coupled to bus 510 forprocessing information and instructions. System 500 also includes datastorage features such as computer-usable volatile memory 530, e.g.random access memory (RAM), coupled to bus 510 for storing informationand instructions for central processor unit 520; computer usablenon-volatile memory 540, e.g. read only memory (ROM), coupled to bus 510for storing static information and instructions for the centralprocessor unit 520; a data storage unit 550 (e.g., a magnetic or opticaldisk and disk drive) coupled to bus 510 for storing information andinstructions; and a network interface unit 590 (e.g. ethernet adaptercard, modem, etc.) for receiving data from and transmitting data to acomputer network. System 500 also includes optional devices such as anoptional alphanumeric input device 570 coupled to bus 510 forcommunicating information and command selections to central processorunit 520; an optional cursor control device 580 coupled to bus 510 forcommunicating user input information and command selections to centralprocessor unit 520; and an optional display device 560 coupled to bus510 for displaying information.

Unless specifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present invention,discussions utilizing terms such as “receiving,” “determining,”“indicating,” “transmitting,” “repeating,” or the like, refer to theactions and processes of a computer system or similar electroniccomputing device. The computer system or similar electronic devicemanipulates and transforms data, represented as physical (electronic)quantities within the computer system's registers and memories, intoother data, similarly represented as physical quantities within thecomputer system memories, into other data similarly represented asphysical quantities within the computer system memories, registers, orother such information storage, transmission, or display devices.

In the embodiments described above, LCD color temperature is altered byselectively dimming the brightness of one or the other of the lightsources so that their overall contribution matches the desired LCD colortemperature. LCD color temperature is also dependent upon thepercentages of different phosphors that are used within the CCFLs. Forinstance, a CCFL tube that includes 33% of R-phosphor, 33% ofG-phosphor, and 33% of B-phosphor will have different color temperature,chromaticity, and brightness than another one that includes 40%R-phosphor, 40% G-phosphor, and 20% B-phosphor. Therefore, it isnecessary to select an appropriate percentage composition of R, G, and Bphosphors such that the color temperature, luminance, and chromaticityof the LCD may be accurately controlled.

According to one embodiment of the present invention, an appropriatepercentage composition of phosphors is selected such that, within apredetermined range of color temperature, the brightness of the LCD isnot reduced below a given threshold minimum (e.g., 70 percent of themaximum brightness). In addition, in the present embodiment, theappropriate mix of R, G, B phosphors is selected such that, within thepredetermined color temperature range, the color temperature of thedisplay is held close to the black body curve 200 of the CIEchromaticity diagram 190 (FIG. 5). Finally, in the present embodiment,the appropriate mix of R, G, B phosphors in the light sources isselected such that their maximum brightness point is set to be near themiddle of the predetermined color temperature range.

FIG. 15 is a flow diagram 600 illustrating a process (executed on system500) of selecting appropriate light source candidates such that, withina predetermined range of color temperature, the brightness of the LCD isnot reduced below a given threshold minimum (e.g., 70 percent of themaximum brightness). For simplicity, in the present embodiment, theselection is made for a “blue” light source 136, and a “red” lightsource 132 of a color balancing embodiment 160 of FIG. 3B. Further, atarget color temperature range is predetermined to be between 3,400 K to8,250 K.

At step 610, high-resolution (1 nm) spectral files for particular typesof R, G, and B phosphors are input into computer system 500 and storedat RAM 530 or data storage unit 550. The high-resolution spectral filesfor R, G, and B phosphors may be obtained from manufacturers of thephosphors. Particularly, each spectral file contains energy data of theemission spectrum of one type of phosphor (e.g. R-phosphors, G-phosphorsor B-phosphors). Then, the energy data of the emission spectra of the R,G, and B phosphors are converted to luminance (brightness) data usingthe human eye sensitivity data over the visible emission spectrum. Forsimplicity, in the following discussion, the luminance data for R, G,and B phosphors are labeled R(λ), G(λ) and B(λ), respectively.Significantly, each light source candidate is essentially represented bythe percentages of R-phosphors, G-phosphors, and B-phosphors present inthe light source candidate.

At step 620, a pool of light source candidates are generated by varyingthe amount of different phosphor types. According to the presentembodiment, a 5%-increment scheme in the relative amounts of differentphosphor types is adopted. For example, one light source candidate mayhave a percentage composition of 25% R-phosphors, 35% G-phosphors, and40% B-phosphors. Another light source candidate may have a percentagecomposition of 40% R-phosphors, 40% G-phosphors, and 20% B-phosphors.Further, in the present embodiment, 400 (20×20) light source candidatesare available.

At step 630, the luminance spectrum W(λ) of each of the light sourcecandidates is computed. According to the present embodiment, a “bluish.”luminance spectrum W1(λ) for “blue” light source 136 is calculatedaccording to the following equation:

 W 1(λ)=a 1*R(λ)+b 1*G(λ)+c 1*B(λ),

where a1, b1, c1 correspond to the percentages of red phosphors, greenphosphors, and blue phosphors, respectively, selected for the “blue”light source 136, and where a1+b1+c1=1. According to the presentembodiment, a total number of 400 calculations have to be made for eachof the 400 light source candidates.

Similarly, a “reddish” luminance spectrum W2(λ) for “red” light source132 is calculated according to the following equation:

W 2(λ)=a 2*R(λ)+b 2*G(λ)+c 2*B(λ),

where a2, b2, c2 correspond to the percentages of red phosphors, greenphosphors, and blue phosphors selected for the “red” light source 132,and where a2+b2+c2=1.

At step 640, a light source candidate is matched up with another lightsource candidate to form a candidate pair. In the present embodiment,the total number of light source candidate is 20 for each of the twolight sources 132 and 136. Therefore, a total number of possiblecandidate pairs is 400.

At step 650, a combined luminance spectrum, W3(λ), is computed for theselected candidate pair. The combined luminance spectrum results fromcontributions from “blue” light source candidate and from “red” lightsource candidate, and can be calculated according to the equation:

W 3(λ)=L 1*W 1(λ)+L 2*W 2(λ),

where L1 and L2 represent brightness levels of “blue” light source 136,and “red” light source 132, respectively. According to the presentembodiment, the brightness level Li of the “blue” light source 136, andthe brightness level L2 of the “red” light source 132 may be selectivelyand independently adjusted to modify the color temperature of the LCD.Further, according to the present embodiment, a 5% increment/decrementscheme in the intensity levels L1 and L2 is adopted. Thus, in thepresent embodiment, after discarding redundancy, a total number of 200combined luminance spectrums are calculated for the selected candidatepair.

At step 660, for each of the combined luminance spectrums generated atstep 650 for the selected candidate pair, a luminance value and a colortemperature is calculated. Methods for calculating luminance values andcolor temperatures from luminance spectrums are well known in the art.Therefore, details of such methods are not described herein to avoidobscuring aspects of the present invention. In one embodiment, a maximumluminance value, Lmax, corresponding to the maximum brightness levels(L1=L2=100%) of the “blue” light source candidate and the “red” lightsource candidate, is also calculated.

At step 670, a table for storing the luminance values and colortemperatures associated with the selected candidate pair is constructed,thus forming a color temperature-luminance relationship for the selectedcandidate pair.

At step 680, candidate pairs having luminance values (L) smaller than aminimum luminance threshold (e.g. 70% of Lmax) between the predeterminedcolor temperature range (e.g. between 3,400 K to 8,250 K) are rejected.In this way, a significant number of candidate pairs are rejected andthe simulation time for any subsequent steps is reduced.

At step 690, it is determined whether all the possible combinations ofcandidate light sources have been processed. If it is determined thatall possible combinations of the candidate light sources have beenprocessed, candidate pairs that do not meet the requisite criterion arerejected, and the processed ends. However, if there are possiblecombinations of candidate light sources that have not been processed,steps 610 through 690 are repeated.

FIG. 16 is a flow diagram 700 illustrating a process of selectingappropriate light source candidates such that within the predeterminedcolor temperature range, the color temperature is held close to theblack body curve 200 of the CIE chromaticity diagram 190.

At step 710, a candidate pair is selected from the pool of candidatepairs that have not been rejected. According to the one embodiment, theprocess illustrated in flow diagram 700 is performed after candidatepairs that do not meet the luminance requirement are rejected.

At step 720, the chromaticity values (x, y) for a candidate pair isdetermined from the combined luminance spectrums calculated in step 660.In one embodiment, a different chromaticity value (x, y) is calculatedfor each luminance spectrum for each candidate pair. Methods forcalculating chromaticities from luminance spectrums are well known inthe art. Therefore, details of such methods are not described herein toavoid obscuring aspects of the present invention.

At step 730, the chromaticity values over the luminance spectrums for acandidate pair are compared to the black body curve 200 of chromaticitydiagram 190. In one embodiment, a relationship of color temperature andchromaticity is built, and the relationship is compared to the blackbody radiation curve. Methods for performing statistical comparison fortwo set of data are well known in the art. Therefore, it would beapparent to those of ordinary skill in the art, upon reading the presentdisclosure, that numerous statistical analysis algorithms may beimplemented herein. In one embodiment, the sum of the derivatives arecomputed and compared to other sums of other candidate pairs.

At step 740, if it is determined that the chromaticity values of thecandidate pair is significantly deviated from the black body curve 200,then the candidate pair is rejected.

At step 750, if it is determined that the chromaticity does notsignificantly deviate from the black body curve 200, then the candidatepair remains in the pool.

At step 760, a new candidate pair is selected from the remaining pool ofcandidate pairs, and steps 710-750 are repeated until all the candidatepairs have been processed. Thereafter, at step 770, when all candidatepairs have been processed, the process returns, and the number ofcandidate pairs is further reduced.

FIG. 17 is a flow diagram 800 illustrating a process of selectingappropriate light source candidates such that a maximum luminance occursapproximately in the middle of a given color temperature range.

At step 810, a pair of light source candidates are selected from thepool of candidate pairs that have passed the luminance thresholdrequirement and that closely follow the black body curve 200.

At step 820 of FIG. 17, color temperature values of the selectedcandidate pair are plotted against luminance values of the selectedcandidate pair to provide a color temperature and luminance diagram.Exemplary color temperature and luminance diagrams are illustrated inFIGS. 9B and 9C.

At step 830, it is determined whether a peak luminance point, or maximumluminance point, i.e. when both “blue” and “red” light source candidatesare turned on at a maximum intensity, occurs approximately at the middleof the given color temperature range. In the exemplary color temperatureand luminance diagram of FIG. 9B, peak luminance point 266 occurs atroughly 4700 K. In FIG. 9C, the peak luminance point 276 occurs atroughly 5050 K.

At step 840, if it is determined that the peak luminance point does notoccur approximately at the middle of the given color temperature range,then the candidate pair is rejected. For instance, in the example asillustrated in FIG. 9B, for a predetermined range of color temperaturebetween 3,400 K to 8,250 K, the peak luminance point should occur atapproximately 5825 K. The peak luminance point, however, occurs atapproximately 4700 K for the example as illustrate in FIG. 9B.Therefore, the example as illustrated in FIG. 9B does not satisfy thisrequirement, and will be rejected.

At step 850, if it is determined that the maximum luminance occursroughly in the middle of the predetermined color temperature range, thenthe candidate pair remains in the pool of potential candidates.Thereafter, the process returns.

At step 860, a new candidate pair is selected from the remaining pool ofcandidate pairs, and steps 810-850 are repeated until all the candidatepairs have been processed. Thereafter, at step 870, when all candidatepairs have been processed, the process returns, and the number ofcandidate pairs is further reduced.

According to one embodiment of the present invention, an offset valuemay be added to the luminance spectrum so as to compensate for theyellow shift caused by many of the layers, e.g., the acrylic in thelight pipes, the ultra-violet cured extraction patterns, the DBEF andBEF films, the polarizers, the LCD layer and the color filters, withinan LCD flat panel display system. The offset value, however, is largelydetermined by the experience and skill of the light tube designer, andby empirical experimentation. In addition, light tube designers mayadjust the values of the percentage compositions of the R-phosphors,G-phosphors, and B-phosphors to produce a pair of CCFL tubes that havethe ideal “look and feel.”

Rear Backlighting Embodiments

FIG. 18A illustrates a backlighting embodiment 910 of the presentinvention that positions an array of light sources 132 a-132 b and 136a-136 b directly under the display layer 912 thereby obviating the needfor any light pipes. A diffuser layer 914 is used to diffuse the lightemitted from the light sources to promote light uniformity. If the lightsources are placed near the diffuser layer 914 (e.g., less than one inchaway), then hiding lines may be required. These hiding lines aretypically etched on the diffuser layer and are more numerous near thebody of the light sources. The relative intensities of the light sources(red and blue) can be controlled to perform color temperature balancingas described above in the numerous edge lit embodiments. A rearreflector layer 138 is also used in embodiment 910.

The information display layer 912 (“display layer 912”) is generally anon-emissive modulated display layer. The layer 912 is non-emissivemeaning that it does not generate any original light but rather modulesthe light of another light source (e.g., sources 132 a-b and 136 a-b).The display layer 912 modules light that layer 912 does not generate.Modulation is used to form images on layer 912 thereby conveyinginformation. In one embodiment, display layer 912 is a liquid crystaldisplay layer of the technology shown in FIG. 2A including the layersbetween layer 126 and layer 112. Alternatively, the display layer 912 anelectrophoretic display layer using ion migration for modulation. Layer912 can also be a reflective display layer or a layer having a fixedmodulated design printed or otherwise laid thereon. Layer 912 can alsobe an electromechanical display using shuttered pixels (“windows”) formodulation. Layer 912 can also be a ferroelectric display.

FIG. 18B illustrates another backlighting embodiment 922 similar toembodiment 910 except the rear reflector 920 contains scallops cuttherein and each light source 132 a-132 b and 136 a-136 b is positionedwith its own scallop to increase directed reflection. FIG. 18Cillustrates another backlighting embodiment 930 that is similar toembodiment 922 except that within the reflector layer 936 a pair oflight sources 132 a and 136 can be positioned within a single scallop.

Conclusion

The preferred embodiment of the present invention, a color balancingsystem for a flat panel LCD unit applying variable brightness tomultiple light sources of varying color temperature, is thus described.While the present invention has been described in particularembodiments, it should be appreciated that the present invention shouldnot be construed as limited by such embodiments, but rather construedaccording to the below claims.

What is claimed is:
 1. A flat panel display comprising: a planar lightpipe disposed to provide backlight to a liquid crystal display layer; afirst light source optically coupled to provide a first spectralcomponent of light to said planar light pipe, said first spectralcomponent having a first color temperature and wherein said first colortemperature is below a predefined minimum color temperature; a secondlight source optically coupled to provide a second spectral component oflight to said planar light pipe, said second spectral component having asecond color temperature and wherein said second color temperature isabove a predefined maximum color temperature; and wherein said first andsecond light sources are stacked parallel to each other and positionedalong a same edge of said planar light pipe.
 2. The flat panel displayaccording to claim 1 wherein a color temperature of said backlight ofsaid flat panel display is adjustable by selectively and independentlyvarying brightness of said first light source and said second lightsource.
 3. The flat panel display according to claim 2 wherein saidfirst color temperature is approximately 3,560 K.
 4. The flat paneldisplay according to claim 2 wherein said second color temperature isapproximately 15,000 K.
 5. The flat panel display according to claim 2and further comprising: a rear polarizer of said flat panel display forpolarizing light in a predetermined orientation, said rear polarizerreceiving light from said planar light pipe; a pre-polarizer interposedbetween said planar light pipe and said rear polarizer and on a firstside of said planar light pipe for allowing a first portion of lightpolarized in said predetermined orientation to pass, wherein a secondportion of light is reflected toward a second side of said planar lightpipe; and a reflector for increasing brightness of said flat paneldisplay, said reflector being positioned on said second side of saidplanar light pipe to receive said second portion of light, saidreflector further having a layer of light rephasing material forrephasing said second portion of light into said predeterminedorientation such that a part of second portion of light is allowed topassed through said pre-polarizer and said rear polarizer.
 6. The flatpanel display according to claim 5 wherein said light rephasing materialcomprises a PTFF material.
 7. The flat panel display according to claim5 wherein said pre-polarizer comprises a layer of dual brightnessenhancement film (DBEF).
 8. The flat panel display according to claim 5further comprising: a light diffusing film disposed between saidpre-polarizer and said rear polarizer for eliminating off-axis defects.9. The flat panel display according to claim 8 wherein said lightdiffusing film comprises a LEXAN film.
 10. The flat panel displayaccording to claim 5 further comprising a crossed brightness enhancementfilm (BEF) interposed between said planar light pipe and said rearpolarizer of said liquid crystal display screen for expanding a viewangle of said liquid crystal display screen.
 11. The flat panel displayaccording to claim 5 wherein said light rephasing material comprises amaterial including barium sulfate.
 12. A flat panel display comprising:a planar light pipe for providing backlight to a liquid crystal displaylayer; a bi-directional light extraction pattern on one surface of saidplanar light pipe for uniformly distributing light over said planarlight pipe; a first light source optically coupled to provide a firstspectral component of light to said planar light pipe, said firstspectral component having a first color temperature that is below apredefined minimum color temperature; a second light source opticallycoupled to provide a second spectral component of light to said planarlight pipe, said second spectral component having a second colortemperature that is above a predefined maximum color temperature; andwherein said first and second light sources are stacked parallel to eachother and positioned along a same edge of said planar light pipe. 13.The flat panel display according to claim 12 wherein a color temperatureof said backlight of said flat panel display is adjustable between saidminimum and said maximum color temperature by selectively andindependently varying brightness of said first light source and saidsecond light source.
 14. The flat panel display according to claim 12wherein said first color temperature is approximately 3,560 K.
 15. Theflat panel display according to claim 12 wherein said second colortemperature is approximately 15,000 K.
 16. The flat panel displayaccording to claim 12 and further comprising: a rear polarizer of saidflat panel display for polarizing light in a predetermined orientation,said rear polarizer receiving light from said planar light pipe; apre-polarizer interposed between said planar light pipe and said rearpolarizer and on a first side of said planar light pipe for allowing afirst portion of light polarized in said predetermined orientation topass, wherein a second portion of light is reflected toward a secondside of said planar light pipe; and a reflector for increasingbrightness of said flat panel display, said reflector being positionedon said second side of said planar light pipe to receive said secondportion of light, said reflector further having a layer of lightrephasing material for rephasing said second portion of light into saidpredetermined orientation such that a part of second portion of light isallowed to passed through said pre-polarizer and said rear polarizer.17. The flat panel display according to claim 16 wherein said lightrephasing material comprises a PTFF material.
 18. The flat panel displayaccording to claim 16 wherein said pre-polarizer comprises a layer ofdual brightness enhancement film (DBEF).
 19. The flat panel displayaccording to claim 16 further comprising: a light diffusing filmdisposed between said pre-polarizer and said rear polarizer foreliminating off-axis defects.
 20. The flat panel display according toclaim 19 wherein said light diffusing film comprises a LEXAN film. 21.The flat panel display according to claim 16 further comprising acrossed brightness enhancement film (BEF) interposed between said planarlight pipe and said rear polarizer of said liquid crystal display screenfor expanding a view angle of said liquid crystal display screen. 22.The flat panel display according to claim 16 wherein said lightrephasing material comprises a material including barium sulfate. 23.The flat panel display according to claim 12 wherein said bidirectionallight extraction pattern comprises: a plurality of extraction dots thatincrease in size as a function of their distance away from said lightsources; and a plurality of relatively larger extraction dots positionedat corners of said extraction pattern.
 24. A flat panel displaycomprising: a planar light pipe for providing backlight to a liquidcrystal display layer, a bidirectional light extraction pattern on onesurface of said planar light pipe for uniformly distributing light oversaid planar light pipe; a first light source optically coupled toprovide a first spectral component of light to said planar light pipe,said first spectral component having a first color temperature; a secondlight source optically coupled to provide a second spectral component oflight to said planar light pipe, said second spectral component having asecond color temperature; and wherein said first and second lightsources are stacked parallel to each other and positioned along a sameedge of said planar light pipe.
 25. The flat panel display according toclaim 24 wherein a color temperature of said backlight of said flatpanel display is adjustable between a minimum and said maximum colortemperature by selectively and independently varying brightness of saidfirst light source and said second light source.